Advances in image processing software and hardware have created a demand for graphic arts electronic imaging systems that are capable of reading or writing high quality images at high data rates. Electronic image writing systems are predominantly required to generate repetitive collinear straight lines with good pixel-to-pixel uniformity and no pixel dropouts. The image writing device of choice for these performance requirements is a flying spot laser scanning system, since the beam can be focused to a fine spot to achieve the high pixel density required for high-resolution imaging applications and the intensity of only a single source need be controlled. Most flying spot laser scanning systems are based on either a galvanometer, a rotating penta prism, a rotating polygonal mirror, a rotating holographic optical element (hologon), or an acousto-optic or electro-optic deflector.
Rotating mirror beam deflector systems did not initially meet the requirements for graphic imaging applications because of facet-to-facet non-uniformities and/or spinner wobble. The problem with mirror facets is that they double every error, whether it be deflector wobble, facet pyramidal alignment error, or facet surface nonflatness. Thus, a single faceted rotating mirror beam deflector, termed a monogon or monofacet, suffers from cross-scan beam error due to changes in the fixed mirror wobble angle caused by motor bearing inaccuracies and/or vibration. U.S. Pat. No. 4,475,787 issued Oct. 9, 1984 to Starkweather shows that cross-scan beam error due to these deflector deficiencies can be eliminated in a preobjective deflector system (scan lens follows deflector) if the light undergoes two reflections in the deflector element. Starkweather shows that this reflection condition is achieved by using a rotating penta prism, penta mirror, right angle prism or right angle mirror monogon.
The rays emerging from the penta prism maintain their angular orientation independent of the angular change in the penta prism fixed wobble angle since the rays undergo two reflections within the penta prism deflector. These two reflections occur from reflecting surfaces which are rigidly coupled to each other and, therefore, experience the same angular deviation as the entire penta prism orientation is changed so that parallel rays emerging from the penta prism are focused by a lens after the deflector to a single point at the focal plane of the lens, which in a recording (printers) system is made coincident with the surface at which the recording media is located.
U.S. Pat. No. 4,853,709 issued Aug. 1, 1989 to Stein et al., shows a penta prism deflector in an internal drum imaging system. The penta prism deflector is used in the postobjective mode (focusing lens before deflector), and offset displacement introduced into the position of the emerging rays from the penta prism due to the change in the penta prism wobble angle, introduces a corresponding offset displacement in the scan beam position (cross-scan beam error) at the image recording plane.
One drawback with the two reflection monogon and penta prism method is that each mirror surface of the device must be about twice as flat as that used in the single reflection surface monogon in order to achieve the same wavefront accuracy. If an average wavefront accuracy of .lambda./4 is needed, each mirror surface must be made to a surface accuracy of approximately .lambda./16, which is expensive. Other major disadvantages of the penta prism deflector include: rotationally nonsymmetric optical and mass geometry that presents problems in a rotating environment, and a fairly large deflector mass that can contribute significantly to inducing dynamic changes in deflector wobble angle. Dynamic change in deflector wobble is still a concern in the penta prism deflector because even though its cross-scan beam error is essentially insensitive with regard to this change, its in-scan beam error (jitter) is directly proportional to change in the in-scan component of deflector wobble.
The rotationally nonsymmetric geometry of the penta prism deflector makes it essentially impossible to obtain dynamic balance in all planes of the deflector. This inability to balance out each deflector plane makes it very difficult to operate the deflector at high rotation rates since the deflector assembly is very prone to vibration problems as a function of change in deflector rotation rate. For example, a penta prism deflector assembly in which mass (a mounting hub) may be added to the assembly to make the assembly more rotationally mass symmetric and while improved with respect to mass symmetry when compared to the penta prism alone still has residual mass asymmetry and, therefore, cannot be completely dynamically balanced for each deflector assembly plane. Also, there is more mass than the initial penta prism deflector which adds considerably to the size, complexity and cost of the deflector motor, and limits speed of rotation of the deflector.
Centrifugal-force-induced deflector element flatness distortion is a problem in mirror deflectors used at high rotation rates. This is particularly true for the penta prism deflector element due to the presence of the two reflecting surfaces and the asymmetric arrangement of those reflecting surfaces with respect to the deflector rotation axis. The second reflecting surface of the penta prism is particularly susceptible to centrifugal-force-induced flatness distortion because it is located at a relatively large distance from the rotation axis and is supported on only one side.
Several other variations of the two reflection deflector principal have been proposed and/or developed. McGrath U.S. Pat. No. 4,934,780, issued Jun. 19, 1990 shows that the two reflection principle can be achieved in a deflector shaped in the form of a 90.degree.-45.degree.-45.degree. prism rotated about an axis that bisects the 90.degree. apex prism angle. With this deflector configuration one achieves mass rotation symmetry and two scans per rotation. Unfortunately, this deflector configuration is not optically symmetric and does not have the incident beam collinear with the deflector rotation axis and, therefore, cannot be used for internal drum imaging applications. Also, it requires a large prism member relative to the optical aperture requirement, and it is not well-suited for operation at high rotation rates.
Articles by G. F. Marshall SPIE Proceedings, Vol. 1454 Beam Deflection & Scanning Technologies (1991), page 37 and Laser Focus World, Vol 27, p.167 (May 1991) describe what is called a butterfly scanner which utilizes two reflections, is mass rotationally symmetric, and produces two scans per rotation. This deflector like the 90.degree.-45.degree.-45.degree. prism is optical nonsymmetric, and the incident beam is noncollinear with the rotation axis. Thus the butterfly deflector cannot be used for internal drum imaging systems. It also requires a large deflector element relative to the optical aperture requirement and is not suited for operation at high rotation rates.
An article by L. Beiser in SPIE Proceedings, Vol. 1454, p. 33 (1991) describes what the author terms an open-mirror monogon scanner that utilizes two reflections to make its cross-scan beam error insensitive with regard to deflector wobble. This deflector has neither optical or mass symmetry, but does have the incident beam collinear with the deflector rotation axis. While this deflector configuration may have a lower mass than a penta prism deflector for the same optical aperture requirement, it suffers from most of the problems that hindered the performance of the penta prism deflector for high rotation rate applications; i.e., balancing problems and centrifugal-force-induced deflector element flatness distortion which translates into optical scan beam distortion.
Tashiro, Japanese Patent Publication 2-226111 of Sep. 7, 1990, describes a two reflection, polarization sensitive, monofacet beam deflector (see FIG. 1) that incorporates a polarizing beam splitter cube, a quarter-wave retardation plate 12 and a high reflecting mirror surface 14. The incident P polarized beam enters the beam deflector element through its top surface and propagates to the polarization sensitive beam splitter reflecting surface 16 that is sandwiched between two essentially identical 45 degree right angle prism elements 18 and 20. This polarization sensitive beam splitter reflecting surface has the property that, at a specific wavelength, it transmits virtually 100 percent of P polarized light while reflecting virtually 100 percent of S polarized light.
After passing through the polarization sensitive beam splitter reflecting surface 16, the P polarized incident beam propagates through the quarter-wave plate 12 to the mirror surface 14. The quarter-wave plate and reflecting mirror surface are orientated perpendicular to the incident beam propagation direction and, therefore, the incident beam is retroreflected back on itself. For illustration purposes, the retroreflected beam 22 in FIG. 1 is shown as propagating at a small angle with respect to the incident beam propagation direction.
Due to the retroreflection condition, the incident beam effectively propagates through the quarter-wave plate 12 twice, thereby experiencing the phase retardation associated with a half-wave plate. When the quarter-wave plate is oriented with its optical axis at 45 degrees to the incident beam polarization direction, the retroreflected beam emerging from the quarter-wave plate will be orthogonally polarized with regard to the incident beam polarization direction (P-in-S out) as shown in FIG. 1. The S polarized retroreflected beam propagates back to the polarization sensitive beam splitter reflecting surface 16 where it is essentially totally reflected in a direction perpendicular to the deflector rotation axis 24 and emerges from the deflector element as the output scanning light beam. In Tashiro's description of this deflector configuration, the output scanning beam enters an F-Theta scan lens and is imaged to a flat image plane.
While the polarization beam splitter and quarter-wave retroreflector arrangement enables the Tashiro device to achieve virtually 100 percent radiometric throughput efficiency, the arrangement introduces a number of performance problems and restrictions with regard to imaging system applications.
A major performance problem associated with the Tashiro deflector configuration is that the polarization sensitive beam splitter reflecting surface and the quarter-wave plate in this deflector assembly are very wavelength dependent with regard to their intended operating characteristics and, therefore, the deflector can only be utilized with a monochromatic light source. This wavelength restriction prevents this deflector from being used for color image recording applications requiring multi-wavelength light sources to write on color photographic film. Even when used with a monochromatic light source, the output scanning beam intensity from this deflector varies as a function of relative changes in the incident beam polarization state.
It is evident from the preceding description of the polarization sensitive beam splitter reflector surface 16 in FIG. 1 that P polarized light is virtually 100 percent transmitted while S polarized light is virtually 100 percent reflected and, therefore, the scan beam intensity is very dependent on the initially incident beam polarization state. Also, this polarization sensitivity property of the deflector produces scan beam intensity variation as a function of the deflector rotation angle, .theta..sub.R, when a linearly polarized light source is used with the deflector. The relationship between scan beam intensity, I, and deflector rotation angle is: EQU I=I.sub.S sin.sup.2 .theta..sub.R +I.sub.P cos.sup.2 .theta..sub.R,(1)
where I.sub.S and I.sub.P are, respectively, the intensities of the S and P polarization components of the incident beam. When deriving Equation (1) it was assumed that the radiometric throughput efficiency of the deflector was essentially 100 percent for P polarized light when .theta..sub.R =0, as depicted in the FIG. 1 deflector configuration.
Equation (1) shows that the scan beam intensity for the FIG. 1 deflector configuration decreases as cos.sup.2 .theta..sub.R. This scan beam intensity decrease with deflector scan angle is not a significant problem for flat-field imaging systems since F-Theta scan lens considerations usually limit deflector rotation angle to a maximum of .+-.27 degrees and, therefore, the intensity fall-off can be compensated for by electronically changing the modulation intensity of the scan beam as a function of scan angle. However, this scan beam intensity fall-off with scan angle is a significant problem for internal drum imaging systems because the majority of these systems use deflector rotation angles of between .+-.85 and .+-.135 degrees.
Tashiro proposes to rotate the laser light source with the deflector assembly as a way to solve the problem of the scan intensity decreasing as a function of deflector rotation angle. This solution is not practical in most imaging applications, particularly those requiring a high deflector rotation rate. In addition, rotation of the laser source with the deflector assembly makes the cross-scan beam error of the laser/deflector assembly sensitive to changes in assembly wobble angle and, thereby, effectively cancels the scan beam error performance achieved by having two reflections within the deflector element.
Other device properties that militate against the use of the Tashiro deflector configuration for internal drum imaging applications include problems associated with utilizing this deflector in the postobjective mode and the perpendicularity of the scan beam with regard to the deflector rotation axis. Both the polarization sensitive beam splitter reflecting surface and the quarter-wave plate are very incident angle dependent with regard to their intended operating characteristics and, therefore, are limited for use with essentially a collimated incident beam. This beam collimation condition precludes the use of this deflector in the postobjective mode, thereby complicating its incorporation into internal drum imaging systems.
It is desirable in an internal drum imaging system to have the scan beam be slightly offset from being perpendicular to the rotation axis so that the retroreflected specular light from the internal drum image surface does not propagate back along the incident beam and cause ghost scan beams and laser intensity instability. For the internal drum imaging configuration, scan-line straightness and image spot velocity uniformity are independent of the angle that the scan beam makes with respect to the deflector rotation axis. These imaging parameters depend on the accuracy of the concentricity between the deflector rotation axis and the internal drum recording surface. A five degree deviation angle between the scan beam and image surface normal is often selected because this ensures that retroreflected specular light from the image surface does not reenter the focusing lens, even for the largest designed scan beam ray cone angle. Image resolution is degraded by utilizing too large of a deviation angle between the image surface normal and the scan beam principal ray. This degradation occurs because the image spot becomes elliptically shaped in the cross-scan direction and because of an increase in image flare associated with multiple reflection of the skewed incident scan beam within the recording medium.
All of the previously described two reflection deflectors have very poor aerodynamic forms which can, at high rotation rates, introduce significant air turbulence into the scan beam path. This air turbulence can significantly increase the scan jitter of the system and, therefore, the deflector element is usually enclosed in an aerodynamically smooth housing. See U.S. Pat. No. 4,662,707 issued to Teach, et al. on May 5, 1987 and U.S. Pat. No. 4,988,193 issued Jan. 29, 1991 to Cain. The addition of this aerodynamic housing usually introduces other unwanted optical properties into the system.
Reference may be had to C. J. Kramer, U.S. Pat. No. 4,852,956 for a nondisc plane diffraction grating (NPDG) monofacet deflector providing accurate, essentially no cross-scan beam tracking error and which can be used in internal drum and flat-field imaging applications. This grating facet redirects the incident laser beam propagating along the deflector rotation axis so that it exits the deflector unit approximately perpendicular to the rotation axis. Rotation of the deflector unit causes the redirected beam from the deflector to scan through an angle that is equal to the deflector rotation angle. Following the grating facet in FIG. 6 of the Kramer Patent is a single element lens that rotates with the deflector unit, thereby enabling the deflector unit to generate high resolution images on the inside surface of a drum. The stationary F-Theta scan lens following the deflector unit in FIG. 7 of the Kramer Patent images the scan beam from the unit to a scanning spot that generates a straight scan line on a flat imaging surface. NPDG deflectors can be substituted for the bulky penta prism deflectors used in many imaging applications. These NPDG deflectors are significantly less affected by centrifugal-force-induced optical beam distortion than penta prism deflectors because the NPDG deflectors function in transmission. Also, the optical and mass symmetry of the NPDG monograting deflector unit enables it to be easily driven to very high rotation rates. However, grating based deflectors require very monochromatic light sources and laser diodes, now commercially available, are not suitable for use with these deflectors due to wavelength shifts associated with mode hopping in these lasers.